Multiphase thermoelectric alloys and method of making same

Abstract

There is disclosed new and improved multiphase thermoelectric alloys and a method for making the same. The alloys are disordered materials having a multiplicity of matrix crystallites separated by generally disordered grain boundaries containing transitional phases and grain boundary regions of various phases including electrically conductive phases having at least one phase having high electrical conductivity. The alloys are formed from a mixture of at least two separately prepared multiple element compounds preferably a first compound Bi.sub.10 Sb.sub.30 Te.sub.60 or Bi.sub.40 Te.sub.48 Se.sub.12 and a second compound Ag.sub.25 Sb.sub.25 Te.sub.50. These compounds while crystalline, have different crystalline structures. They themselves are polycrystalline and do not represent the most stable crystalline structure. The first compound has a rhombohedral crystalline structure and the second compound has a face centered cubic crystalline structure. The compounds are combined in solid particulate form in proportions of 97 to 99.25 percent Bi.sub.10 Sb.sub.30 Te.sub.60 and 3 to 0.25 percent Ag.sub.25 Sb.sub.25 Te.sub.50 or 99 percent Bi.sub.40 Te.sub.48 Se.sub.12 to 1 percent Ag.sub.25 Sb.sub.25 Te.sub.50. The mixture is then heated in a quartz tube to an elevated temperature and then drawn through a temperature gradient for cooling. The alloys include in the grain boundary regions various phases of silver and tellurium. The silver containing phases establish low resistance current paths through the crystallites to provide the alloy with high electrical conductivity. The disorder of the grain boundaries and the non-highly electrical conductive phases of the grain boundary regions provide low thermal conductivity desired for thermoelectric applications. Also disclosed are alloys doped with a dopant such as tellurium iodide to form thermoelectric alloys having maximized S.sup.2 .sigma. products.